Renal osteodystrophy: histologic evaluation after renal transplantation

Improvement of Mineral and Bone Disorders After Renal Transplantation

BACKGROUND

Posttransplant mineral and bone diseases are causes of fractures, and their association with cardiovascular events is being studied.

METHODS

We analyzed the evolution of biochemical, histological, and imaging parameters pre- and 1 y post-renal transplantation in 69 patients and correlated mineral and bone findings with coronary calcifications. At inclusion and after 12 mo, clinical data and echocardiographic findings were recorded, and laboratory evaluations, radiography of the pelvis and hands, and bone biopsy were performed. Noncontrast cardiac computed tomography was performed during the second evaluation.

RESULTS

Serum levels of fibroblast growth factor 23 and sclerostin decreased in all patients, parathyroid hormone levels decreased in 89.8% of patients, bone alkaline phosphatase levels decreased in 68.1% of patients, and alpha-Klotho levels increased in 65.2% of patients. More than half of the patients presented with renal osteodystrophy at both biopsies, but histological findings improved: a significant transition from high to normal or low turnover and no significant differences in volume, mineralization defect, or cortical porosity at the 2 evaluations. Alpha-Klotho, sclerostin, and bone alkaline phosphatase shifts affect bone changes. Neither echocardiographic findings nor vascular calcification scores differed between the 2 points. Both the pretransplant period (dialysis vintage, sclerostin, and low bone volume at baseline) and the maintenance of abnormalities in the posttransplant period (high turnover posttransplant) were the most reliable predictors of the severity of the coronary calcification percentile.

CONCLUSIONS

Renal transplantation improved bone and mineral abnormalities. The pretransplant period determines the severity of calcification.

Bone densitometry versus bone histomorphometry in renal transplanted patients: a cross-sectional study

Bone loss leads to increase risk of fractures in renal transplantation. The aim of this study was to analyse the relationship between bone densitometry (DXA) findings, bone histomorphometry and bone-related molecules 1-year after renal transplantation. We performed a cross-sectional study of de novo renal transplanted patients that agreed to perform a bone biopsy and a DXA examination 1 year after transplantation. All patients underwent a laboratory evaluation, bone biopsy, DXA examination and cardiac CT 1 year after transplantation. 67 patients were included, 16 had a normal examination, and 18 patients were classified as having osteoporosis by DXA. Correlations between bone mineral density and T-scores of total femur and femoral neck were the ones that best correlated with bone volume assessed by a bone biopsy. The sensitivity of DXA for osteoporosis diagnosis was 47.0%, and the specificity was 81.2%. The positive predictive value was 50.0%, and the negative predictive value (NPV) was 80.0%. DXA parameters also correlated with klotho and sclerostin serum levels. In this population, a normal examination excluded the presence of osteoporosis, helping in identifying patients that would not benefit from therapy. Overall, densitometry in total femur and femoral neck correlated well with bone volume measured by bone biopsy.

Recovery versus persistence of disordered mineral metabolism in kidney transplant recipients

In patients with end-stage renal disease, successful renal transplantation improves the quality of life and increases survival, as compared with long-term dialysis treatment. Although it long has been believed that successful kidney transplantation to a large extent solves the problem of chronic kidney disease-mineral and bone disorders (CKD-MBD), increasing evidence indicates that it only changes the phenotype of CKD-MBD. Posttransplant CKD-MBD reflects the effects of immunosuppression, previous CKD-MBD persisting after transplantation, and de novo CKD-MBD. A major and often-underestimated problem after successful renal transplantation is persistent hyperparathyroidism. Besides contributing to posttransplant hypercalcemia and hypophosphatemia, persistent hyperparathyroidism may be involved in the pathogenesis of allograft dysfunction (nephrocalcinosis), progression of vascular calcification, and bone disease (uncoupling of bone formation and bone resorption and bone mineral density loss) in renal transplant recipients. Similar to nontransplanted patients, CKD-MBD has a detrimental impact on (cardiovascular) mortality and morbidity. Additional studies urgently are needed to get more insights into the pathophysiology of posttransplant CKD-MBD. These new insights will allow for a more targeted and causal therapeutic approach.

Effect of kidney transplantation on bone

A broad range of different factors aggravates renal osteodystrophy, which is present in virtually all patients with chronic kidney disease and after successful kidney transplantation. Altered hormonal status, including sex hormones and parathyroid hormone (PTH), a deficit of 1,25(OH)(2) vitamin D(3) (calcitriol), immunosuppressive therapy and post-operative immobilization contribute to a progressive loss of bone density and structure. The decrease of bone mass is particularly prominent during the first 6 months after kidney transplantation and is associated with an increased number of fractures, both compared with the normal population as well as with dialysis patients. At particular risk are patients with a history of diabetes, long duration of haemodialysis and post-menopausal women. To prevent post-transplant bone loss prescription of steroids should be minimized and withdrawn as early as possible. Additional intake of alpha-calcidol [25(OH) vitamin D(3)] or calcitriol, despite normal serum levels, reduces persistent hyperparathyroidism after kidney transplantation, improves intestinal calcium absorption and activates osteoblasts. Inhibition of osteoclasts by biphosphonate therapy seems to effectively reverse bone loss during the early and late course of kidney transplantation. However, as the majority of transplant recipients have a low-turnover bone disease, inhibition of osteoclasts, through which bone turnover is impaired, might further reduce osteoblast activity and promote osteoid synthesis. Most investigations were small-scale studies with 10-100 participants and a follow up of only 12 months. This makes conclusions on the effect of any intervention on the fracture rate impossible. Larger, randomized multicentre studies investigating bone-sparing therapy on hard end points are therefore advocated.

Bone disease after renal transplantation

Bone disease is common after renal transplantation. The main syndromes are bone loss with a consequent fracture rate of 3% per year, osteonecrosis of the hip, and bone pain. The causes of disease include preexisting uremic osteodystrophy (hyperparathyroidism, aluminum osteomalacia, beta2-associated amyloidosis, and diabetic osteopathy), postoperative glucocorticoid therapy, poor renal function, and ongoing hyperparathyroidism, as the result of either autonomous transformation of the parathyroid gland or ongoing physiologic stimuli. Cyclosporine A treatment, hyperphosphaturia, and a pathogenic vitamin D allele have also been implicated. Bone loss is particularly pronounced during the first year after operation, amounting to up to 9% of bone mass. The clinical and biochemical picture is consistent with a high turnover bone disease, but histomorphometric studies do not completely support this. Principal prophylactic options include preoperative osteodystrophy prophylaxis; postoperative calcium, vitamin D, or calcitriol therapy; estrogen therapy for postmenopausal women; and parathyroidectomy for medically intractable hyperparathyroidism. Recently, prophylactic biphosphonate treatment has shown promise, but the exact indications for treatment remain to be determined.